Traditionally, individual moving coil and cone loudspeakers are placed within the doors, instrument panel and rear tray and elsewhere in a vehicle for providing sound within the vehicle. These speakers add substantial weight to a vehicle, require individual installation and connection, occupy valuable interior trim space, allow significant road noise intrusion, and are subject to substantial shock and environmental abuse.
Most significantly, they are poorly positioned for listening. Their on-axis radiation is typically directed low in the vehicle toward occupants legs and midsections rather than at the occupants ears. The direct sound from the speaker to the listener is typically far off-axis and highly variable in frequency response with typically insufficient high frequencies. In the high noise environment of a vehicle, this typically results in mid and high frequency audio information getting lost. “Imaging”, the perception of where sound is coming from, is also adversely affected since the loudspeakers are low in the vehicle; for the front passengers, the audio image is pulled down into the doors while the rear passengers have an image to the side or rear instead of what should be presented in front of them.
As a solution to this problem, some proposed systems, including the system described in the U.S. patent to Clark et al. U.S. Pat. No. 5,754,664, have incorporated small, lightweight loudspeaker drivers above the occupants in the headliner in addition to the door and rear package tray speakers. Unfortunately, the small loudspeaker can still be localized due to the fact that the listener is far enough to be in the free field of acoustic radiation but not far enough to be experiencing a plane wave condition.
This phenomenon, as documented by Soren Bech and others results in an unnatural simulation of an acoustic space. S. Bech, “Electroacoustic Simulation of Listening Room Acoustics. Psychoacoustic Design Criteria”, AUDIO ENGINEERING SOCIETY, 89th Convention 21–25 Sep. 1990, Los Angeles, USA, 34 pp. The most significant drawback of this approach, however, is that the overall system complexity and cost is increased due to the addition of individual drivers overhead while the conventional speakers still remain in the doors and rear package tray. Furthermore, the noise paths through the door and rear package trays still exist and more noise paths from the roof (as occurs in rain) are opened with the new lightweight cone speakers in the headliner. Lastly, making the drivers invisible would be extremely difficult, since the small speakers are mounted onto the headliner; even if acoustically transparent fabric were placed over the drivers, the holes in the headliner would result in “read-thru” or visibility.
The Verity Group PLC has applied for a number of patents covering various aspects of flat panel loudspeaker (i.e., NXT) technology. The technology operates on the principle of optimally distributive modes of vibration. A panel constructed in accordance with this technology has a very stiff structure and, when energized, develops complex vibrations mode over its entire surface. The panel is said to be dispersive in that the shape of the sound wave traveling in the panel is not preserved during propagation.
Unfortunately, distributed mode panel loudspeakers require precise geometries for exciter placement and panel suspension thus limiting their size and integration capabilities into a headliner. Essentially, they would be separate speakers assembled into a hole in the headliner or onto the surface of the headliner. In the first case, they would also result in extra noise transmission (since the panels are extremely light) or in the second case, they would be visible to the occupants either as bumps or edges in typical headliner covering materials. In both cases, added complexity is the result.
From a sonic performance viewpoint, distributed mode panels suffer from poor low frequency response (typically restricted to 250 Hz and above for sizes integral to a headliner) and low output. Neither of these conditions make NXT panels suitable for headliner applications, particularly in a high noise environment. Furthermore, distributed mode panels are incapable of precise imaging, presenting instead a diffuse acoustic field perception where the sound appears to come from everywhere. While distributed mode panels might improve overall spaciousness, they would still require full range loudspeakers in the doors or rear package tray for sufficient acoustic output and other speakers in front for proper imaging.
In the U.S. patent to Parrella et al. U.S. Pat. No. 5,901,231, driving portions of interior trim with piezoelectric elements to reproduce audio frequencies is disclosed. However, the use of piezo-electric elements restricts them to dividing up the trim into different sections for different frequency ranges adding complexity to the system. Furthermore, the excursion limits of piezo elements limits the output level and low frequency range of the trim panels such that conventional cone speakers would be required to produce lower frequencies. The piezo elements also require complicated integration into the trim element and are difficult to service. Lastly, the piezo elements require additional circuitry to convert typical output from an automotive head unit further complicating the system.
The above-noted application entitled “Integrated Panel Loudspeaker System Adapted To Be Mounted In A Vehicle” describes flat panel systems with an electromagnetic drive mechanism integrated into an aperture in the panel. However, the driving mechanism that is integrated into the panel is constructed without steel pieces to contain, direct and concentrate the magnetic flux to its best advantage. The voice coil required is also relatively massive severely limiting the high frequency output. Thus, the output level is not adequate for typical audio performance. Furthermore, the aperture that the electromagnetic drive mechanism is insufficiently stiff to produce high frequency output.
The U.S. patents to Marquiss U.S. Pat. Nos. 4,385,210, 4,792,978 and 4,856,071 disclose a variety of planar loudspeaker systems including substantially rigid planar diaphragms driven by cooperating coil and magnet units.
Thus, even with the above prior advancements in flat speaker technology and overhead audio, prior audio systems have not been simplified. There is still a need to reduce parts and labor cost, decrease weight, decrease exterior noise penetration, provide believable imaging, reduce speaker visibility, increase reliability, and provide easy serviceability.
It is therefore desirable to provide an audio system which achieves the above by using existing trim panel space and mounting techniques, conventional audio signal head unit output, advanced material properties manipulation and well established signal processing, and psychoacoustic techniques.